1. Field of the Invention
This invention relates to the solvent extraction of butadiene from a mixture of hydrocarbons having four carbon atoms per molecule (C4's).
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce individual olefin products such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In such olefin production plants, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to severe hydrocarbon thermal cracking at elevated temperatures of about 1,450 to 1,550° Fahrenheit (F.) in a pyrolysis furnace (steam cracker or cracker).
The cracked product effluent from the pyrolysis furnace contains hot, gaseous hydrocarbons, both saturated and unsaturated, of great variety from 1 to 35 carbon atoms per molecule (C1 to C35). This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate product streams of high purity, e.g., molecular hydrogen, ethylene, and propylene. After separation of these individual streams, the remaining cracked product contains essentially hydrocarbons with four carbon atoms per molecule (C4's) and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5 and heavier stream is removed as a bottoms product.
The crude C4 stream has a variety of compounds such as n-butane, isobutane, 1-butene, 2-butenes (cis and trans), isobutylene, butadiene (1,2- and 1.3-), vinyl acetylene, and ethyl acetylene, all of which are known to boil within a narrow range, U.S. Pat. No. 3,436,438. Further, some of these compounds can form an azeotrope. Crude C4's are, therefore, known to be difficult to separate by simple distillation.
The crude C4 fraction, after removal of acetylenes, normally goes to a butadiene extraction unit for separation of butadiene from the fraction. Thereafter, isobutylene can be removed by, for example, reaction with methanol to form methyl-tert-butyl ether (mtbe). Butenes can then be distilled from the mtbe, and 1-butene separated from 2-butenes by simple distillation.
The dominating process for separating butadiene from crude C4's is known technically as “fractional extraction,” but is more commonly referred to as “solvent extraction” or “extractive distillation.” However it is termed, this process employs an aprotic polar compound that has a high complexing affinity toward the more polarizable butadiene than other olefins in the crude C4 stream. Known solvents for this process include acetonitrile, dimethylformamide, furfural, N-methyl-2-pyrrolidone, acetone, dimethylacetamide, and the like. This process and the solvents used therein are well known, U.S. Pat. Nos. 2,993,841 and 4,134,795. It is equally well known that this type of process inherently generates internally tars (tar) that, if not controlled, can affect the quality of the butadiene separated as a product of the process, and even plug equipment, thereby causing an expensive and time consuming shut down and clean out of the plant. Accordingly, there is continuous effort in the industry to which this process pertains to find solvents that reduce tar formation and deposition in equipment.
This invention takes a different tack from industry in addressing the control of tar formation and deposition in a butadiene extraction unit, in that it controls tars without changing the known solvents used in such a process.
Heretofore, in butadiene extraction plants such as that shown in FIGS. 1-4 herein below, wherein a primary solvent and a secondary solvent were employed, it was dogma that some tar content suspended in the solvent mixture (primary and secondary) circulating in the system was necessary to keep tar formation and deposition at a minimum in the system as a whole. Accordingly, operators of such extraction plants were required without fail to maintain in the solvent mixture a tar level (load) of not less than 2 weight percent (wt. %) and a total content of tars plus secondary solvent of 5 wt. %, both weight percents being based on the total weight of the solvent mixture plus tar circulating in the system. For example, the unswerving operating specifications for this type of plant known as the Nippon-Zeon design required the tar level to be 2 wt. % minimum and the combined tar and secondary solvent level to be 5 wt. %, i.e., 2 wt. % tar and 3 wt. % secondary solvent, the remainder being 95 wt. % primary solvent. These design criteria were slavishly followed by operators of such plants.